A thread wound golf ball is a golf ball comprising a core (liquid core or solid core), a thread rubber layer formed by winding a thread rubber around the core in a stretched state and a cover covering the thread rubber layer.
Generally, it is necessary that the golf ball has suitable impact resilience so as to obtain excellent flight performances. Particularly, the thread wound golf ball requires a thread rubber having excellent impact resilience, because impact resilience and flight performances of the ball is extremely influenced by the properties of the thread rubber layer.
A thread rubber having excellent impact resilience can be obtained by vulcanizing a rubber blend comprising a low-cis polyisoprene rubber as a base rubber, however, the low-cis polyisoprene rubber is inferior in mechanical strength and heat resistance. The thread rubber formed from the low-cis polyisoprene rubber imparts disadvantage to the resulting golf ball. That is, the thread rubber is liable to be damaged on forming the thread rubber layer by winding around the core at the stretched state, which is the cause of deterioration of durability of the ball. Further, heat upon forming the cover damages the thread wound layer, thus deteriorating ball compression.
Therefore, in order to improve the strength of thread rubber, it has been proposed to blend natural rubber or synthesized high-cis polyisoprene rubber in with the low-cis polyisoprene rubber. However, impact resilience of the resulting golf ball adversely declines, although its strength is improved. It is therefore difficult to obtain both impact resilience and high durability.
On the other hand, in the case of using a method of preparing the thread rubber from a coagulated sheet of a latex blend, a kneading process wherein breakage of molecular chain occurs is unnecessary. Therefore, the above disadvantage in strength is slightly improved. However strength and impact resilience thereof are still insufficient.
As described above, in the thread wound golf ball, impact resilience of the ball is extremely influenced by the properties of the thread rubber constituting the thread rubber layer, whereby flight performances of the ball are extremely influenced. According to the prior art, when impact resilience is increased, strength is lowered and, therefore, the thread rubber having both high strength and excellent impact resilience were not obtained.